A permanent-magnet synchronous motor (hereinafter, simply “motor”, except when specific distinction is necessary) has been known as a highly efficient motor, and has the following characteristics: the motor does not require an excitation current because a magnetic flux is established based on a permanent magnet incorporated in a rotor, as compared with conventional induction motors widely used in various fields; and any secondary copper loss does not occur in the motor because a current does not flow to a rotor like the induction motor. While an induction motor is also conventionally used in electric vehicles, application of a permanent-magnet synchronous motor has been examined in recent years to improve its efficiency.
Generally, in an electric vehicle that runs with plural cars connected to each other, each incorporating a power conversion device and a motor, the electric vehicle can continue running by using sound power conversion devices and motors, even when a line to line fault occurs in a power conversion device in a part of the cars during running. As a result, because the motor connected to a power conversion device having the fault is kept driven at a wheel side, a short-circuit current continues flowing at an induced voltage of the motor at a fault portion (a line to line fault portion) of the power conversion device having a short-circuit fault.
Therefore, when this state is left as it is, there is a risk that the damage of the fault portion of the power conversion device further advances due to the heat by a short-circuit current or causes heat or burn of the fault portion or the motor, and this state is not desirable.
As a countermeasure against such cases, for example, Patent Document 1 discloses the following method. A contactor as a switching unit that electrically disconnects the connection between an inverter and a motor is provided to avoid expansion of a damage of the inverter at an induced voltage of the motor, when the inverter in a power conversion device that drive-controls a permanent synchronous motor is in a fault during running of an electric vehicle. When a controller detects a fault of the inverter, the controller open-circuit-controls the contactor, and electrically disconnects the inverter from the motor.    Patent Document 1: Japanese Patent Application Laid-open No. H8-182105
As is generally known, a sinusoidal alternating current generates a current zero point at each half cycle of a current wave. Therefore, a current can be interrupted by using the current zero point. The contactor described in Patent Document 1 mentioned above is an alternating current disconnecting contactor that interrupts a current by using the current zero point. Generally, types of the contactor to interrupt an alternating current include a vacuum contactor and the like using a system of interrupting a current at a current zero point.